1. Field of the Invention
The present invention relates to a device for determining the state of electricity generation of a solar battery for use in an apparatus such as a camera.
2. Description of the Related Art
FIG. 8 is a block diagram showing the circuit arrangement of a conventional device provided with the function of detecting the electricity generating capability (the state of electricity generation) of a solar battery.
The circuit arrangement shown in FIG. 8 includes a solar battery 51 composed of a plurality of solar cells connected in series, a secondary battery 52, such as a lithium-ion battery, for storing the electrical energy generated by the solar battery 51, an overcharging preventing circuit 53 for protecting the secondary battery 52 from being overcharged with the electrical energy generated by the solar battery 51, a diode 54 which serves as a reverse-current preventing element for preventing electric current from flowing back to the solar battery 51, and diodes 55 and 56 which serve as similar reverse-current preventing elements.
The circuit arrangement shown in FIG. 8 also includes a voltage detecting circuit 57 composed of an amplifier 57a, an A/D converter 57b and the like. The voltage detecting circuit 57 serves to detect an electric current which flows through a resistance 58 located in a charging loop for charging the secondary battery 52, and convert the detected electric current into an output voltage. The shown circuit arrangement also includes a CPU 59 for performing display control of a display 60 (which will be described later) in accordance with the output of the voltage detecting circuit 57, and the display 60 for visually displaying the electricity generating capability of the solar battery 51. The display 60 is arranged to visually display the electricity generating capability by means of, for example, four segments SEG1 to SEG4 as shown in FIG. 9.
In the above-described circuit arrangement, the voltage across the resistance 58 located in the charging loop for charging the secondary battery 52, which voltage corresponds to the electricity generating capability of the solar battery 51, is amplified by the amplifier 57a provided in the voltage detecting circuit 57 and is then A/D-converted by the A/D converter 57b. The obtained voltage information is outputted from the A/D converter 57b to the CPU 59. When receiving this voltage information, the CPU 59 drives and controls the display 60. In this manner, the user can readily know the electricity generating capability of the solar battery 51. FIG. 9 shows the state in which all the segments SEG1 to SEG4 are turned on to indicate that the electricity generating capability of the solar battery 51 is sufficiently high.
However, the above-described conventional device involves a number of problems. For example, the resistance 58 is located in the charging loop for charging the secondary battery 52 with the electrical energy generated by the solar battery 51, and the voltage produced across the resistance 58 is temporarily amplified by the amplifier 57a, such as an operational amplifier, provided in the voltage detecting circuit 57. This arrangement consumes an electric current of approximately several milliamperes because the amplifier 57a needs to be operated.
If a predetermined amount of electrical energy is not stored in the secondary battery 52, electric power is supplied from only the solar battery 51 to operate the voltage detecting circuit 57. During this time, if the electricity generating capability of the solar battery 51 is smaller than the rate of current consumption of the amplifier 57a, the circuit becomes unable to detect the voltage across the resistance 58 and to provide a visual display indicative of the detected voltage level.
One approach to the above-described problems is to adopt an arrangement for detecting the voltage across the resistance 58 by only the A/D converter 57b without using the amplifier 57a, as shown in FIG. 10. In this arrangement, however, since only a small charging current flows through the resistance 58, it is necessary to increase the resistance value of the resistance 58 so as to generate a voltage equivalent to the voltage amplified by the amplifier 57a. In other words, it is necessary to set the resistance value of the resistance 58 to (resistance value.times.amplification magnification) times the resistance value that is required when the amplifier 57a is used. Therefore, a resistance having a large value needs to be used in the charging loop for charging the secondary battery 52 with the generated electrical energy, and the charging efficiency is degraded when a large amount of charging current is applied.